user already exists error in php PDO

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PHP

hy to every one.. iam new in php and working with PDO interaction to mysql. iam going to filter user input data using

if(!filter_input(INPUT_POST,'$username')||!filter_input(INPUT_POST,'$password'))
        {
            echo "username or password already exists....";

        }

        else
        {
$sql = "INSERT INTO users (Username,Password) VALUES (:username,:password)";
$q=$dbh->prepare($sql);
$q->execute(array(':username'=>$username,':password'=>$password));
$count=$q->rowCount();
echo "inserted $count rows.\n";

but it show an error... the error is that it simply show the message

username or password already exists

if the username or password is not exists it show the same message? what can i do? if some one have any idea then put it to my code... if the filter_input() is not used her then what can i use insted of... plz solve it... thank's in Advance..